The present invention relates to a photoconductor element for use in electrophotographic imaging and, in particular, to a barrier layer for a photoconductor element. The invention also relates to a method of producing a photoconductor element having a barrier layer.
The formation and development of images through electrophotographic imaging is well known. One electrophotographic imaging process involves the sequential steps of charging a photoconductor element, usually with a high voltage corona, forming an electrostatic latent image with laser exposure, developing the image by applying toner particles thereto to form a visible toner image corresponding to the electrostatic latent image, and transferring the toner image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. The toner particles may be dispersed in either a dry or liquid medium, and may form black and white or full color images. Heat and pressure are often used to facilitate image transfer from the photoconductor element to the substrate.
The photoconductor element may take the form of a continuous belt which is supported and circulated by rollers, or may be adhered to the outer surface of a rotatable drum. Further, many different constructions exist for the photoconductor element. Common to all such constructions is a photoconductive layer which is formed from a material which acts as an insulator except when exposed to light. That is, the photoconductive layer does not conduct an electric current unless it is being exposed to light. Various organic and inorganic materials exist from which the photoconductive layer may be formed.
The photoconductive layer is generally affixed to and supported by an electroconductive support. The electroconductive support may be either negatively or positively charged such that when light strikes the photoconductive layer, electrons either flow from the electroconductive support and through the photoconductive layer (in the case of negatively charged electroconductive support), or through the photoconductive layer and into the electroconductive support (in the case of a positively charged electroconductive support).
In addition to the photoconductive layer and electroconductive support, other layers may be included in the photoconductor element. For example, a release layer topcoat may be included on the uppermost surface of the photoconductor element. This layer is constructed from a material having a low surface energy and serves to increase the efficiency with which toner particles are transferred from the surface of the photoconductor element. Silicone and fluorocarbon polymers have been previously described as effective for release layer applications.
Another type of layer which may be included in the photoconductor element is a barrier layer. A barrier layer may be positioned between the photoconductive layer and the release layer to protect the photoconductive layer. In this manner, the barrier layer enhances the durability and extends the service life of the photoconductive layer. To be effective in this capacity, the barrier layer should ideally meet many different performance criteria. First, the barrier layer should protect the photoconductive layer from damage due to corona-induced charge injection.
Such damage reduces the useful life of the photoconductive layer, and can be caused by the corona when placing a charge upon the surface of the photoconductor element. Damage occurs when the charge is permitted to directly contact the photoconductive layer. The corona also creates ozone and ionized particles which can further damage the photoconductive layer if permitted to directly contact that layer. Ozone, ionized particles, and charge from the corona are believed to damage the photoconductive layer by directly or indirectly causing unwanted reactions with the photoconductive layer, e.g., oxidation. An effective barrier layer is one which can prevent or substantially minimize direct contact of the photoconductive layer by the ozone, ionized particles, and charge which are produced by the corona.
A second requirement of the barrier layer is that it should be substantially inert with respect to the photoconductive layer. That is, the barrier layer should not chemically react with the photoconductive layer to the extent that the performance of the photoconductive layer is detrimentally affected and "trap sites" form between the barrier layer and the photoconductive layer. Trap sites are localized electrical voids which can retain electrons as the electrons attempt to flow through the photoconductor element in the areas which have been light struck, e.g., from a negatively charged electroconductive support to neutralize a positively charged surface of a photoconductor element, the positive charge being placed there by a corona (the inverse case is also possible where the electroconductive support is positively charged and a negative charge is placed upon the surface of the photoconductor element by the corona). Current cannot flow between the support and the surface until a sufficient number of electrons are retained in the trap sites to provide a conductive path therethrough. Until the trap sites are adequately filled with electrons in this manner, the surface of the photoconductor element cannot be adequately discharged when exposed by a light source because a sufficient number of electrons will not be able to flow between the electroconductive support and the surface to neutralize all of the charge in the light struck areas.
Trap sites thus result in long warm-up (or burn-in) periods before the photoconductor element is ready to produce high quality toner images. During such warm-up periods, the photoconductor element is repeatedly charged by the corona and discharged by a light source in order to allow the trap sites to fill with electrons. Any images produced before adequate warm-up will be of poor quality due to insufficient toner particle attraction in the image-wise exposed areas. Moreover, when the imaging device housing the photoconductor element is turned off, the trap sites may become emptied once again. Thus, the next time the imaging device is turned on, another warm-up period is required. This problem is known as "reset."
A third requirement of the barrier layer is that it should adhere well to the photoconductive layer and the release layer without the need for adhesives. Fourth, when the photoconductor element is used in belt form, the barrier layer should exhibit sufficient resiliency to withstand the compressional and tensional forces exerted on the photoconductor element as it travels around the aforementioned system of rollers.
A fifth requirement exists when liquid toner systems are used. Liquid toner systems generally include toner particles dispersed in a carrier liquid, and may include other constituents such as charge control agents. The barrier layer should be capable of substantially limiting or preventing the liquid toner system from coming into contact with the photoconductive layer. The toner particles, carrier liquid, and other constituents can damage and/or shorten the service life of the photoconductive layer.
Although many barrier layer formulations have been proposed, none have been found which are capable of satisfactorily meeting all of the above-stated performance criteria. For example, U.S. Pat. Nos. 4,439,509, 4,606,934, 4,595,602, and 4,923,775 disclose a protective overcoating utilizing a cross-linkable siloxanol-colloidal silica hybrid material. The siloxanol-colloidal silica hybrid material is generally formed by combining a trialkoxysilane, a colloidal silica hydrosol, and an organic acid. U.S. Pat. No. 4,439,509 discloses that the siloxanol-colloidal silica hybrid material is cross-linked with ammonia gas to form the protective overcoating. U.S. Pat. No. 4,606,934 discloses that when no organic acid is used to form the siloxanol-colloidal silica hybrid material, an acid number of less than 0.5 can be achieved. U.S. Pat. No. 4,595,602 discloses that the siloxanol-colloidal silica hybrid material is combined with a hydrolyzed ammonium salt of an alkoxy silane to produce the protective overcoating. U.S. Pat. No. 4,923,775 provides that the siloxanol-colloidal silica hybrid material is combined with a silane having an electron accepting moiety.
Each of above-described protective overcoatings as recited in U.S. Pat. Nos. 4,439,509, 4,606,934, 4,595,602, and 4,923,775 result in photoconductor elements which require relatively long warm-up periods, and which often "reset" such that long warm-up periods are required each time the respective photoconductive imaging device is turned on. These problems are believed to result from the formation of trap sites between the overcoating and the photoconductive layer due to chemical reaction between the overcoating and the photoconductive layer.
Moreover, the above overcoatings do not provide the photoconductive layer with adequate protection from liquid contact when a toner dispersed in a carrier liquid is utilized. In addition, such overcoatings provide insufficient resiliency to be used in belt form. When tested as such, the overcoatings developed stress fractures.
U.S. Pat. No. 4,565,760 discloses a protective overcoating formed from a dispersion of hydroxylated silsesquioxane and colloidal silica in an alcoholic medium. Such an overcoating, however, suffers from the same deficiencies as those described immediately above (i.e. long warm-up periods, reset problems, poor liquid toner carrier protection, and insufficient resiliency).
U.S. Pat. No. 5,124,220 discloses a photoconductor element having a barrier layer and a release layer. The barrier layer is formed from an organic polymer such as one resulting from a mixture of polyvinyl alcohol with methylvinylether/maleic anhydride copolymer. This barrier layer provides good carrier liquid protection but does not protect the photoconductive layer from corona-induced charge injection and ozone/ionized particle creation.
Accordingly, a need exists in the art for a barrier layer which meets all of the above-listed performance criteria, i.e., one which provides charge injection protection, is substantially inert with respect to the photoconductive layer, adheres well to the photoconductive layer and to the release layer, exhibits sufficient resilience to be used in belt form, and protects the photoconductive layer from contact with toner carrier liquid.